This invention relates to an ignition timing control method for internal combustion engines at idle, and more particularly to a method of this kind which is adapted to control the ignition timing of the engine at idle so as to accurately maintain the engine rotational speed at a desired idling speed.
It is desirable for an internal combustion engine to rotate stably at a desired idling speed appropriate to an idling condition in which the engine operates. However, actually, it is sometimes difficult to maintain the desired idling speed at a constant value due to fluctuations in engine load.
Amongst attempts to solve this problem, it is known e.g. from Japanese Provisional Patent Publication No. 52-153042 to control the rotational speed of an internal combustion engine at idle by advancing or retarding the ignition timing, to thereby maintain the desired idling speed constant. According to this conventional ignition timing control method, feedback control is effected in such a manner that actual engine speed is detected, the ignition timing is retarded when the detected engine speed is above a desired idling speed, and the ignition timing is advanced when the detected engine speed is below the desired idling speed. However, since this control method advances or retards the ignition timing in a feedback manner responsive only to actually detected engine speed, ignition timing control is effected irresponsive of engine coolant temperature. As a result, when the engine is operating in a cold state where combustion within engine cylinder can often be instable, fluctuations in the engine speed can occur accordingly. Therefore, even if the ignition timing is advanced or retarded in the above-mentioned feedback manner with the engine in such a cold state, it is difficult to accurately control the actual engine speed to the desired idling speed. Further, the conventional control method is not purely electronic type, suffering from a detection delay in detecting the engine conditions and a control delay, which can cause hunting in the engine rotation, rendering it difficult to maintain the engine speed constant even when the engine coolant temperature is not so low. Moreover, according to the conventional control method wherein the ignition timing is varied in an idling condition of the engine to effect idling speed feedback control as described above, the ignition timing always varies, which makes it difficult to carry out adjustment of the ignition timing at the time of delivery of the engines from the plant as well as that at the time of maintenance operation, in a simple manner, requiring a great deal of time to complete the adjustment.